In the prior art there has been considerable effort expended in extending the spectral bandwidth, or wavelength range, of a broad class of optical metrology instruments. Non-contact, optical measurements are heavily utilized in the optics, optical communications and semiconductor industries. The instruments are used in the evaluation and characterization of samples that can include spatially non-uniform distributions of a broad class of materials including, insulators, semiconductors and metals; consequently, the optical properties of these samples can vary markedly with wavelength. Hence, in general, a much greater wealth of information can be extracted from broadband spectroscopic measurements than can be obtained from measurements made over a narrow spectral range. One approach to implementing broadband spectroscopic measurements is set forth in U.S. Pat. No. 6,278,519, which is incorporated herein by reference.
At present, leading-edge industrial lithography systems operate in the DUV over a narrow wavelength region of approximately 193 nm. State-of-the art optical metrology systems that operate over the spectral range spanning the DUV-NIR (190 nm–850 nm) characteristically employ two lamps to span this range of measurement wavelengths, a Deuterium lamp for spectroscopic measurements between 190 and 400 nm, and a Xenon lamp for measurements between 400 nm and 800 nm.
In the near future systems will operate in the vacuum ultra-violet at an exposure wavelength of 157 nm. Wavelengths in the range between 140 nm–165 nm lie within a region known as the vacuum ultraviolet (VUV), in which the high absorption coefficients of oxygen and water vapor lower the attenuation length in standard air to fractions of a millimeter. (Historically, this light could only be observed under vacuum conditions, hence the designation.) Achieving the transmission and stability necessary for optical metrology in the VUV, in a tool where the optical paths are of order 0.5–2 m requires oxygen and water concentrations in the low parts-per-million (ppm) range averaged over the entire optical path. One approach to achieving this is described in copending U.S. application Ser. No. 10/027385, filed Dec. 21, 2001, now U.S. Pat. No. 6,813,026, and incorporated herein by reference, which discloses a method for inert gas purging the optical path.
The technology extension requires that the measurement bandwidth of optical metrology tools be broadened to cover the wavelength range spanning 140–850 nm. This is a daunting challenge.
First, optical systems that spectrally segregate the illumination with diffractive elements must account for and suppress unwanted signals produced by harmonic contamination. For example, at wavelength λ a diffraction grating will produce a 1st order an intensity maximum at an angular position θ; at a wavelength λ′=λ/2 the same grating, in the 2nd order, also produces an intensity maximum at θ. Consequently, when 140 nm light impinges upon a diffractive grating in addition to the 1st order diffracted beam, there are 2nd, 3rd, 4th, 5th and 6th etc. diffractive orders that appear at angular positions corresponding to 1st order diffraction at 280 nm, 420 nm, 560 nm, 700 nm and 840 nm light, etc., respectively. Consequently spectrometers that use diffraction gratings to disperse the light must, at a minimum, include order-sorting, optical filters to suppress contributions from the higher orders.
Second, most common optical materials undergo solarization, structural and electronic changes to the material that occur upon exposure to VUV and DUV light. VUV and DUV exposure can significantly modify and degrade material optical properties. To avoid the adverse effect of solarization, refractive VUV optical systems must exclusively employ wide bandgap optical materials such as CaF2, MgF2, LiF, and LaF3.
Third, virtually all optical materials are dispersive; i.e. the material refractive index varies as a function of wavelength, or optical frequency. Therefore, the focal position of an individual lens will be wavelength dependent giving rise to chromatic aberration of the lens. This phenomenon complicates the design of broadband optical systems. Chromatic correction can be achieved, to a limited extent, using lenses with multiple optical elements fabricated from at least two optical materials. The general idea is to select and configure components so that there is partial cancellation of the chromatic effects, thereby increasing the useful wavelength range. The designs may be very complex and involve the use of multiple components arranged in multiple groups. The complexity of the optical design and the cost of system fabrication increase with the optical bandwidth.
Note that, to some extent, requirements two and three above are mutually exclusive. It may not be practical to provide a lens design that simultaneously exhibits good transmission in the VUV and chromatic correction over the 140 to 850 nm wavelength range. As proposed herein, one solution to this problem is to spectrally segregate the broadband light and provide multiple lens systems. Each lens operates over a limited wavelength range, or optical sub-band. The sum of the sub-bands constitutes the overall system bandwidth. Here, each lens is designed to operate over a limited wavelength range. This may permit a simpler optical design to be used in each of the sub-band lenses and provide a system architecture that provides superior broadband optical performance at reduced system cost.
Accordingly it would be desirable to provide a metrology tool architecture that permits extension of the measurement bandwidth over the spectral region spanning the VUV-NIR 140 nm–850 nm and avoids the problems associate with atmospheric absorption, chromatic aberration and harmonic contamination.
The acronyms used in this specification have the following meanings:                CD=critical dimension        DUV=deep ultraviolet        VUV=vacuum ultraviolet        NIR=near infrared        